This invention relates to an apparatus for measuring the clearance between a recording transducer and a recording medium, such as a magnetic recording medium, and a recorder using the apparatus, and a method of controlling the recorder.
Particularly, this invention relates to a head clearance measuring apparatus which is suitable for measuring the clearance of the magnetic head over a mounted magnetic recording medium of various types, and suitable for measuring a very small head clearance in a short time and at a high accuracy.
The invention further relates to a recorder which uses the above-mentioned head clearance measuring apparatus for controlling the clearance between the recording head and recording medium, thereby making a smaller head clearance for the achievement of higher density recording, and is highly adaptable to extensive shapes of a recording medium surface, and the invention also relates to a control method for the recorder.
For high density recording of a magnetic recorder, the clearance (aerodynamic lift) of the recording transducer over the recording medium needs to be small. Conventionally, the head clearance has been measured on the basis of the light interference or the variation of electrostatic capacity, or by using a laser doppler velocity meter. However, it is not possible or is very difficult for these methods to measure a head clearance of 0.1 .mu.m (100 nm) or less.
In recorders which operate with a clearance maintained between the recording transducer and recording medium, a reduced clearance can result in a higher recording density. One method for positioning the recording transducer to have a constant clearance against extensive shapes of the recording medium surface is to cover the medium surface with a conductor and fit an electrode on the recording transducer carrier, so that the head clearance is controlled using a tunnel current flowing between the transducer electrode and recording medium conductor, as disclosed in Japanese Patent Unexamined Publication No. 62-125521, for example.
Another method uses an actuator provided between the head carrier and the head so as to produce a relative movement between these members in conjunction with a contact sensor or electrostatic capacity sensor, thereby controlling the clearance between the recording transducer and recording medium, as disclosed in Japanese Patent Unexamined Publication No. 62-250570, for example.
In the case of the conventional magnetic head clearance measuring apparatus based on the light interference, when the head clearance becomes a quarter wavelength or less of the measuring light, interference fringes disappear, making accurate measurement difficult. The measurement of displacement based on the variation of electrostatic capacity or laser doppler is useful for the measurement of the head clearance over the recording medium, but it cannot reveal the absolute clearance value.
A recent proposal for the head clearance control uses the tunneling phenomenon (Japanese Patent Unexamined Publication No. 62-125521). This method offers the measurement of an extremely small clearance of 1 nm or less between two conductors confronting each other in a vacuum or near-vacuum environment. This method, however, requires both of the recording medium and electrode to have a conductive surface, and therefore it cannot be applied to the head clearance measurement over a magnetic disk whose surface is in most cases coated with a nonconductive lubricant or protection film. Namely, the method is not suited to measure the head clearance over a magnetic disk which is already mounted for operation.
A crucial factor in the measurement of an extremely small head clearance is the shape of the recording medium surface. In this respect, the method based on the tunnel current is restricted in the type of recording medium. The tunnel current, which is inversely proportional to the exponent of the distance between conductors multiplied by a constant, is approximately 1 nA at a 1 nm distance, and the current decreases sharply as the distance increases. The current value is less related to the shape of electrode, since only couples of atoms in a shortest distance contribute to the tunnel current dominantly, leaving other atoms more distant by one atom diameter (0.1-0.3 nm) to produce a significantly diminished tunnel current. On this account, in order to increase the intensity of tunnel current by varying the shape of electrode, the electrode needs to be finished as precise as a fraction of 1-atom dimension, and it is very difficult and costly. Accordingly, the detection, measurement and assessment of a tunnel current flowing between two conductors spaced out by several nanometers or more is only feasible by using an expensive apparatus.
A tunnel current is as small as 1 nA and therefore susceptible to electrical noise. In addition, a current created by a varying capacitance caused by a varying clearance between the tunnel electrode and recording medium can influence significantly the signal current. Moreover, the voltage for maintaining the tunneling phenomenon is as low as 0.1 volt, and it is also susceptible to disturbances in the electromagnetic environment.
The conventional techniques have encountered the problems of head clearance measurement of the order of 0.1 .mu.m in the above-mentioned respects. The foregoing techniques do not comprehend the situation in which the clearance between the recording transducer and recording medium varies to the extent beyond control. In case of using a contact sensor or electrostatic capacity sensor, as mentioned above, the head clearance is measured in terms of the electrostatic capacity which is proportional on the first-order basis to the distance between the confronting conductors, and therefore it merely provides a same output variation for a same distance variation even if the distance to be measured becomes small, which imposes difficulty of control for a smaller clearance measurement in principle.
The following describes the general concept of field electron emission in relation to the foregoing prior art problems.
A necessary condition for the field electron emission to arise is to make an electric field to the extent of about 10.sup.8 V/m or more (the field strength depends on the material and surface condition of the conductor) on the surface of a conductor which has a negative voltage value with respect to the surrounding. Generally, field electron emission is induced in a vacuum by application of several thousand volts to the sharp tip of a metallic needle so that a large electric field is produced at the needle tip. FIG. 1 explains the mechanism of field electron emission. In the figure, when a strong electric field 102 is produced on the surface of a conductor 101 at the tip of a metallic needle, the electric field 102 penetrates into a conductor surface region 104, causing the energy potential distribution outside of the conductor and in the conductor surface region 104 against free electrons 103 in the conductor to vary from the state shown by the dashed line 105 to the state shown by the solid line 106. Consequently, a very thin energy barrier 107 attributable to the work function emerges, and the free electrons 103 in the conductor penetrate the energy barrier 107 by the tunneling phenomenon based on quantum mechanics (Schottky tunnel effect) to form a field electron emission current 109 into the free space as shown by the arrow 108. Alternatively, as shown in FIG. 2, when two conductors 111 and 112 are spaced out by a gap 113, with a potential difference being made between the conductors (shown as a step 115 of the energy potential distribution of free electrons in the conductors 111 and 112 in the figure), a large electric field 116 is produced. The electric field causes the energy potential distribution to vary from the state shown by the dashed line 117 to the state shown by the solid line 118. A thin energy barrier 120 is created in a surface region 119 of the conductor 111 having a negative voltage, and if the potential difference between the two conductors is greater than the work function of the conductor 111, the Schottky tunnel effect 108 emerges to produce a field electron emission current 109 (in FIG. 1, a lower step of electron potential distribution in the conductor and those shown by 121 and 122 in FIG. 2 are steps attributable to the work function of the conductor).
Another method of inducing the quantum mechanical tunneling phenomenon in a small gap between two conductors is as shown in FIG. 3. In this case, conductors 124 and 125 are disposed with a small gap 123 provided between them, and a voltage is applied so that a potential energy difference 128 smaller than work functions 126 and 127 of both conductors shown by the electron potential distribution 137 is created. When the gap 123 is sufficiently small (typically 1 nm or less), the energy barrier 131 created from the surface region 129 of conductor 124 to the gap 123 and to the surface region 130 of conductor 125 becomes sufficiently thin, causing free electrons 133 in the conductor to penetrate as shown by the arrow 132 due to the quantum mechanical tunneling phenomenon to produce a tunnel current 134. In this case, however, if one of conductors is covered on its surface with a nonconductor dielectric) 135 as shown in FIG. 4, an energy barrier 136 having a virtually infinite height is created at a portion of the dielectric 135 at a midway point of the energy barrier 131, and the quantum mechanical tunneling phenomenon subsides and the tunnel current does not flow.
On the other hand, in the case of using the field electron emission effect, even in the presence of a dielectric substance 138 on the surface of the conductor 112 which does not have field electron emission, as shown in FIG. 5, an electric field 139 as strong as the electric field 116 of FIG. 2 is produced on the surface of the conductor 111, provided that the conductors 111 and 112 are given such a potential difference that the potential difference between the conductor 111 and the surface of dielectric 138 is greater than the work function of the surface of conductor 111, resulting in the occurrence of field electron emission as in the case of FIG. 2. Shown by the dashed line 140 in FIG. 5 is the electron energy potential distribution of FIG. 4 presented for comparison. The intensity of the current increases sharply as an exponential function of the increasing electric field strength on the conductor surface due to an increased potential difference or decreased spacing between the conductors, as will be described later. However, if the spacing is too small as in the case shown in FIG. 6, the depth 142 of a recess 141 of the electron energy potential distribution existing between the conductor 111 and dielectric 138 becomes shallower than the work function 121 of the conductor 111 as shown in FIG. 6, and the quantum mechanical tunneling phenomenon at the energy barrier of the conductor surface region 119 subsides.